This invention relates to a process for dispersing or redispersing a catalytically active noble metal species on a porous inorganic support which itself may or may not possess catalytic activity, e.g., dispersing or redispersing palladium or platinum on a zeolite catalyst such as ZSM-5 which has become deactivated due to the accumulation of carbonaceous material (e.g., coke) during the course of its use in a hydrocarbon conversion operation such as catalytic dewaxing.
Heterogeneous porous inorganic acidic oxides are used extensively in the petroleum and petrochemicals industry to catalyze a variety of hydrocarbon conversions. These conversions include catalytic cracking, hydrocracking, naptha reforming, benzene alkylation, xylene isomerization, catalytic dewaxing, and other conversions.
During use, as is generally know, the catalysts undergo progressive loss of catalytic activity and/or selectivity. The time required for the activity to decay to the point at which the catalyst is no longer useful may vary from as little as a few minutes, as in catalytic cracking, to several years, as with some versions of naptha reforming. Some of the factors which affect the aging rate include the nature of the feed, the nature of the catalyst and process conditions. In general, catalyst deactivation is accompanied by an accumulation of carbonaceous matter on the catalyst, and is was early learned to regenerate deactivated catalysts by burning the carbonaceous matter in an oxygen-containing gas. In the case of metal-loaded catalysts, the severity of the burning operation often leads to agglomeration of the metal component into relatively large crystallites which, of course, are inherently less active than small crystallites which possess a greater surface area for an equivalent amount of metal.
Reactivation of noble metal catalysts utilized in hydrocarbon processing procedures such as reforming is known in the art. Processes which utilize chlorine and oxygen in catalyst reactivation are well known. For example, U.S. Pat. No. 2,906,702 disclosed a method of restoring the activity of an alumina-supported platinum catalyst after deactivation occurring during the reforming of hydrocarbons. According to this method, a deactivated platinum-alumina catalyst is contacted with gaseous chlorine, fluorine, or other halogen or halogen-affording substance at an elevated temperature.
U.S. Pat. No. 3,134,732 describes a method for reactivating noble metal catalyst supported on alumina by contacting the catalyst with halogen-containing gas, stripping excess halogen therefrom and subjecting the resulting catalyst to a reduction step with a hydrogen-containing gas. This treatment is intended to break up the large noble metal crystallites into smaller crystallites.
U.S. Pat. No. 3,201,355 discloses reactivating a deactivated metal oxide-supported noble metal catalyst utilized in hydroforming processes. Reactivation is accomplished under anhydrous conditions employing a gaseous source of halogen such as chlorine (which is preferred) or nitrosyl chloride admixed with an oxygen-containing gas such as air or an inert gas such as nitrogen or carbon dioxide as the reactivating agent.
U.S. Pat. No. 3,625,860 discloses a process for activating and/or reactivating a platinum on alumina reforming catalyst by contacting the catalyst which has been previously subjected to a sequence of oxidative burn-off, oxygen treatment, purging and reducing operations with a nonmetallic chloride-containing compound, e.g., an organic chloride such as tertiary butyl chloride, propylene dichloride, carbon tetrachloride, etc., or an inorganic chloride such as hydrogen chloride.
It is also known in the art to regenerate platinum group metal-containing zeolite catalysts. Regeneration of noble metal-loaded zeolite catalysts requires certain procedural modifications because the metal must be returned in a dispersed form within the zeolite pores.
U.S. Pat. No. 3,986,982 describes a procedure in which deactivated platinum group metal-loaded zeolite is contacted with a stream of inert gas containing from 0.5 to 20 percent volume of free oxygen and from 5 to 500 ppm volume of chloride as chlorine, HCl, or an organic chlorine-containing material. The resulting catalyst is purged to remove residual oxygen and chlorine and then reduced in a stream of hydrogen at 200.degree. to 600.degree. C.
Other processes for regenerating or otherwise treating metal-loaded zeolites, most of which feature the use of molecular chlorine or other source of chlorine, are described in U.S. Pat. Nos. 3,943,052; 4,444,895; 4,444,897; 4,447,551; 4,517,076; 4,518,708; 4,600,700; 4,645,751; and, commonly assigned copending U.S. patent application Ser. No. 819,074, filed Jan. 15, 1986; U.K. patent application No. 2,106,413 and European patent application No. 142,352.
Processes for treating catalysts featuring the use of an oxide of nitrogen are also known.
U.S. Pat. No. 3,243,383 discloses a process for regenerating a spent cobalt oxide on carbon catalyst, useful in olefin polymerization, wherein the spent catalyst, following heating at 250.degree.-1000.degree. C. in an inert atmosphere and cooling, is treated with nitric acid, nitric oxide (NO), nitrogen dioxide gas (NO.sub.2) or mixtures thereof and thereafter ammoniated, if desired, and finally heated to reactivation temperature.
U.S. Pat. No. 3,451,942 describes a process for the rejuvenation of a deactivated hydrocracking catalyst containing a hydrogenation-dehydrogenation component present in the form of large crystallites on an acidic cracking component, e.g., an arsenided-nickel-on-fluorided-silica-alumina catalyst, in which the deactivated catalyst, following removal of at least a major part of the accumulated carbonaceous matter therefrom, is treated with a nitrogen oxide selected from the group consisting of NO, NO.sub.2, N.sub.2 O and N.sub.2 O.sub.3, optionally, in aqueous nitric acid solution, under conditions causing the material to react with the hydrogenation-dehydrogenation component. Thereafter, the catalyst is treated with oxygen which reacts with the hydrogenation-dehydrogenation component followed by reduction of the latter with hydrogen. It is hypothesized that the hydrogenation-dehydrogenation component is sequentially converted in this series of operations to a salt, possibly a nitrate, then, following the treatment with oxygen, into an ionic form and finally, following reduction with hydrogen, into small crystallites.
According to the process for regenerating a supported tellurium and/or tellurium compound-containing catalyst disclosed in U.S. Pat. No. 3,536,631, the deactivated catalyst is treated at 50.degree.-400.degree. C. with a gaseous nitrogen oxide of the formula NO.sub.x in which x is 1, 1.5, 2 or 2.5 and/or a gaseous oxyacid of nitrogen of the formula HNO.sub.y in which y is 2 or 3.
Deactivated phosphomolybdic acid based catalysts which are used for the conversion of saturated and unsaturated aldehydes to acids are reactivated by the process of U.S. Pat. No. 4,471,062 by feeding an oxide of nitrogen, preferably nitric oxide (NO), over the deactivated catalyst at 100.degree.-400.degree. C.
Che et al., "A Study of the Chemisorption of Nitric Oxide on PdY Zeolite. Evidence For a Room Temperature Oxidative Dissolution of Pd Crystallites", J. Phys. Chem. 80, 2371-2381 (1976) describes the redispersion of mildly agglomerated palladium, i.e., crystallites of 20 Angstroms, supported on zeolite Y with nitric oxide at room temperature. It has been observed that in the case where agglomerated noble metal crystallites of at least about 25 Angstroms average diameter, and more usually at least about 100 Angstroms average diameter, are concerned, e.g., a crystallite size which is typical of a supported noble metal catalyst which has experienced a coke burn-off operation, the redispersion procedure described in this publication does not provide consistent results unless a nitric oxide(s) stripping operation (of which no mention is made by Che et al.) is carried out at the conclusion of the nitric acidcontacting operation.
Foger et al., "The Redispersion of Iridium on SiO.sub.2 and Gamma-Al.sub.2 O.sub.3 Supports with Chlorine-containing Gases", J. Catalysis, 96, 154-169 (1985) discloses that while a gaseous mixture containing a major amount of chlorine and a minor amount of nitric oxide is effective for redispersing iridium on alumina, the mixture is not effective for redispersing iridium on silica. Similarly, Foger et al., "Redispersion of Pt-Ir Supported on Gamma-Al.sub.2 O.sub.3 and SiO.sub.2 in Chlorine-Containing Gases" discloses that the aforesaid chlorine-nitric oxide mixture is not effective for redispersing bimetallic Pt-Ir on silica. There is no suggestion in either of these publications of what the effect of using a chlorine-nitric oxide mixture in which the nitric oxide is quantitatively the major reactant would be.